The splicing factor ASF/SF2 is associated with TIA-1-related/TIA-1-containing ribonucleoproteic complexes and contributes to post-transcriptional repression of gene expression

TIA-1-related (TIAR) protein is a shuttling RNA-binding protein implicated in several steps of RNA metabolism. In the nucleus, TIAR contributes to alternative splicing events, whereas, in the cytoplasm, it acts as a translational repressor on specific transcripts such as adenine and uridine-rich element-containing mRNAs. In addition, TIAR is involved in the general translational arrest observed in cells exposed to environmental stress. This activity is encountered by the ability of TIAR to assemble abortive pre-initiation complexes coalescing into cytoplasmic granules called stress granules. To elucidate these mechanisms of translational repression, we characterized TIAR-containing complexes by tandem affinity purification followed by MS. Amongst the identified proteins, we found the splicing factor ASF/SF2, which is also present in TIA-1 protein complexes. We show that, although mostly confined in the nuclei of normal cells, ASF/SF2 migrates into stress granules upon environmental stress. The migration of ASF/SF2 into stress granules is strictly determined both by its shuttling properties and its RNA-binding capacity. Our data also indicate that ASF/SF2 down-regulates the expression of a reporter mRNA carrying adenine and uridine-rich elements within its 3' UTR. Moreover, tethering of ASF/SF2 to a reporter transcript strongly reduces mRNA translation and stability. These results indicate that ASF/SF2 and TIA proteins cooperate in the regulation of mRNA metabolism in normal cells and in cells having to overcome environmental stress conditions. In addition, the present study provides new insights into the cytoplasmic function of ASF/SF2 and highlights mechanisms by which RNA-binding proteins regulate the diverse steps of RNA metabolism by subcellular relocalization upon extracellular stimuli.